The ornamental design for a loop for transformer, as shown and described.
1. Loop for transformer
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Right
1.6 : Top
1.7 : Bottom
1.8 : Rear perspective view
1.9 : Perspective view in environment of use
1.1 is a front perspective view of a loop for transformer showing the new design; 1.2 is a front view thereof; 1.3 is a rear view thereof; 1.4 is a left view thereof; 1.5 is a right view thereof; 1.6 is a top view thereof; 1.7 is a bottom view thereof; 1.8 is a rear perspective view thereof; and 1.9 is another perspective view thereof, shown in an environment of use; the broken lines in the figures depicting portions of the loop for transformer form no part of the claimed design; the broken lines showing a transformer in 1.9 are for showing environment and form no part of the claimed design.